Study of gigantic third-order optical nonlinearity in low-dimensional correlated electron systems

本研究项目的目的是定量测量三阶非线性极化率χ^&lt；(3)&gt；谱，并阐明卤素桥联镍化合物以及一维和二维铜的一维关联电子体系中三阶光学非线性的机理。我们对这些材料进行了几种类型的非线性光谱测量，如扫描测量和透射型三次谐波(THG)测量。主要结果如下：1.在一维铜酸盐和镍化合物中，奇偶激发态是简并的。而在二维铜的激发态中，偶激发态位于比奇激发态稍低的能量处。1D和2D铜酸盐之间的差异与电荷和自旋自由度在1D系统中是分离的，但在2D系统中是耦合的这一事实有关。因此，在一维体系中，基态与奇激发态、奇激发态与偶激发态之间的跃迁偶极矩增强，从而获得较大的χ^(3)&gt；。用反射式THG测量估算的溴桥镍化合物的χ^&lt；(3)&gt；是非常大的，超过10^lt；-8&gt；esu。在该化合物中，光激发态弛豫很快，吸收光谱宽度很小，是一种很有潜力的非线性光学材料。

The purpose of this research project is to measure quantitatively third-order nonlinear susceptibility χ^<(3)> spectra and clarify the mechanism of third-order optical nonlinearity in one-dimensional (1D) correlated electron systems of halogen-bridged nickel compounds and 1D and 2D cuprates. We have performed several kinds of nonlinear spectroscopy measurements on these materials such as -scan measurements and transimission-and reflection-type third harmonic generation (THG) measurements. The main results are as followed.○ In the 1D cuprates and the nickel compounds, odd and even excited states are degenerate. In the 2D cupreates, on the other hand, the even excited state locates at the energy slightly lower than the odd excited state. Such a difference between the 1D and 2D cuprates is related to the fact that charge and spin degrees of freedom are separated in the 1D system, but coupled in the 2D system. In the 1D system, therefore, the transition dipole moments between the ground state and the odd excited state and between the odd and even excited states are enhanced and then the larger χ^<(3)> is obtained○The magnitude of χ^<(3)> of the bromine-bridged nickel compound evaluated by the reflection-type THG measurements is extremely large, exceeding 10^<-8> esu. In this compound, the relaxation of the photoexcited state is very fast, and the spectral width of the absorption is very small, suggesting its high potential as a good nonlinear optical material for applications.